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| Click on the image to enlarge. Photo © Daniel R. Snyder |
The Yellow Dog peridotite, thought to be approximately 1.1 Ga in age, is composed of partly serpentinized peridotite, containing 40 to 50 percent olivine (1/2 to 2/3 serpentinitzed) and 30 percent pyroxenes consisting of subequal amounts of orthopyroxene and clinopyroxene. (Klasner, et al., 1979)*. In addition, it contains 5 to10 percent plagioclase, which would classify it as a "plagioclase-bearing lherzolite" according to the IUGS criteria.
The peridotite body was first described by William Morris in his 1977 M.S. thesis**. He named it the "Yellow Dog Plains peridotite". The surface outcrop is roughly oval-shaped in horizontal cross-section. Doug Hull's comment below is valid. Appendix C (Geology) to the mining development permit request is now on the Web. You can get it in PDF format by Googling "Eagle Deposit Geology". This report includes cross-sections, at irregular horizontal intervals and at 50-meter vertical intervals, of what is referred to as the "Eagle Deposit". These diagrams justifiy calling it a dike.The report states that the "intrusion at the surface" extends 480 meters in length and is 100 meters wide at its thickest point. The peridotite intrudes metasediments that are at least 1.9 Ga in age.
The image above shows two heavily fractured olivine macrocrysts, which were most likely carried upward by magma originating in the upper mantle. A serpentine meshwork fills most of the fractures, and stringers of magnetite occupy the larger fractures in the crystal on the right. Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 2.7 mm x 4 mm.
Below is an optical scan of a fractured surface of a hand sample of Yellow Dog peridotite. 2400 dpi optical scan, Imaged area 17 mm x 17 mm.
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| Click on the image to enlarge. Photo © Daniel R. Snyder |
*John S. Klasner, David. W. Snider, W. F. Cannon, and John F. Slack (1979), The Yellow Dog Peridotite and a Possible Buried Igneous Complex of Lower Keweenawan Age in the Northern Peninsula of Michigan, Geological Survey Division, Michigan Department of Natural Resources, 38 p.
**Morris, William J., (1977) Geochemistry and Origin of the Yellow Dog Plains Peridotite, Marquette County, Northern Michigan, unpublished master's thesis, Michigan State University, 82p.
Oops...I meant to say, "dike". I'm sure you got a smile out of that one! I did.
ReplyDeleteDoug: Thanks for your comments. I suspect you may know more about this than I do. The early publications (Snider, Klasner) refer to a "dike swarm", and Morris in his 1977 M.S. thesis mentions this, but consistently calls the body an "intrusion." Kennecott's permit application has a Geology appendix, but it's omitted from the online version. However, there are some good cross-sections, ostensibly from Kennecott documents, included in the guidebook of the 11th International Platinum Symnposium (2010): 11IPS-LSgdbk.pdf which you should be able to find without difficulty. You be the judge. The text refers to the two "Eagle deposits" as "small, sub-vertical dike-like mafic-ultramafic bodies". I suppose I could clean up this post.
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